1. Field of the Invention
The present invention relates to integrated circuits (ICs) and reference circuits, and in particular, to a bandgap reference voltage circuit.
2. Description of the Related Art
In the design of large-scale integrated circuits, it is often necessary to provide a local reference voltage of a known value that remains stable with both temperature and process variations. A common prior art solution is a bandgap reference circuit. A bandgap reference circuit provides stable, precise and continuous output reference voltages for use in various analog circuits. Recently, it has become necessary for many commercial integrated circuits to operate at less than the conventional five-volt power supply voltage, such as three volts. As a result, bandgap reference voltage circuits must operate over a power supply range from over five volts down to three volts and less. The output reference voltage provided by known bandgap reference circuits, however, typically varies somewhat with respect to one or more of factors, such as temperature and manufacturing processes. Some known bandgap reference circuits fail to function when the power supply voltage is lowered to three volts.
One method of providing a voltage reference is to provide a stable reference current through a precision resistor. The base-emitter voltage VBE of a forward-biased bipolar transistor is a fairly linear function of absolute temperature T in degrees Kelvin (.degree.K), and is known to provide a stable and relatively linear temperature sensor. In a bandgap reference, the reference voltage is obtained by compensating the base-emitter voltage of a bipolar transistor VBE for its temperature dependence (which is inversely proportional to temperature) using a proportional to absolute temperature (PTAT) voltage. The difference known as "delta VBE" or ".DELTA.VBE" between the base-emitter voltages VBE1 and VBE2 of two transistors that are operated at a constant ratio between their emitter-current densities forms the PTAT voltage. The emitter-current density is conventionally defined as the ratio of the collector current to the emitter size. Thus, the basic PTAT voltage .DELTA.VBE is given by: ##EQU1## where k is Boltzmann's constant, T is the absolute temperature in degree (Kelvin), q is the electron charge, J1 is the current density of a transistor T1, and J2 is the current density of a transistor T2. As a result, when two silicon junctions are operated at different current densities (J1, J2), the differential voltage .DELTA.VBE is a predictable, accurate and linear function of temperature.
One conventional low-voltage bipolar bandgap reference having curvature correction is capable of operating when the power supply voltage is lowered to less than three volts. Such low-voltage bandgap reference is described in Gunawan et al., A Curvature-Corrected Low- Voltage Bandgap Reference, IEEE Journal of Solid-State Circuits, Vol. 28, No. 6, June 1983, pp. 667-670, incorporated herein by reference and illustrated in FIG. 1. In low voltage bandgap reference circuit 100, a current proportional to VBE (2IVBE) and a nonlinear correction current (2INL) are generated. When the nonlinear (curvature) correction is performed correctly, 2(IVBE+INL) should consist of a constant component and a component that is proportional to absolute temperature (PTAT). This latter component can be compensated by using a PTAT current source. The sum of the currents is converted into the voltage reference VREF by using a resistor Rref. A buffer circuit BUFF is applied to obtain a sufficiently low output impedance. With such a configuration, the low-voltage reference VREF has the typical attractive feature of bandgap references that the output voltage is temperature-independent when this voltage is adjusted for a predetermined value. The minimum supply voltage is 1 V for an operating temperature range from 0.degree. C. to 125.degree. C. The circuit also operates at temperatures lower than 0.degree. C., but then a slightly higher supply voltage has to be tolerated.
Although this conventional low-voltage bandgap reference circuit 100 can operate at low supply voltages, the circuit 100 only operates with bipolar technology. Thus, a need exists for a bandgap reference circuit that can operate at low supply voltages, uses an adjustable reference voltage, and is not limited to operation in bipolar technology.